Figure 1. Water surface technology vs. traditional soft contact lens material.
Alcon’s water surface technology contact lenses feature a novel permanent two-phase material – a silicone hydrogel (SiHy) material at the core and a hydrogel water layer at the surface (Figure 1).1–3 Dr Jessica Mathew looks at how this compares with traditional soft contact lenses.
The material of Alcon’s water surface technology lenses is quite different from any other traditional soft contact lens on the market. The SiHy core of the water surface technology contact lenses allows for high breathability and ease of handling, while the hydrogel water layer allows the surface of the lens to be soft (low modulus), lubricious, and highly wettable. This is important as the surface layer interacts with the tissues of the eye to help provide comfort. By contrast, traditional soft contact lenses on the market consist of a homogenous material from core to surface†(Figure 1).1–3
While in vitro testing does not show how a lens will perform on-eye, or in one eye to the next, it does allow for impartial comparisons of how the material alone behaves
Testing Lens Material Properties
To demonstrate differences in contact lens material properties, in vitro (off-eye) bench testing is often used to control all the testing conditions and environment, as this is not possible when conducting on-eye testing. A few examples of in vitro measures commonly used in industry include lens surface wettability, surface lubricity, modulus, and oxygen permeability (Dk).
While in vitro testing does not show how a lens will perform on-eye, or in one eye to the next, it does allow for impartial comparisons of how the material alone behaves. This type of in vitro testing is ideal because it allows the conditions of all lenses being tested to be precisely controlled in the same manner to reveal how the materials truly compare to each other without interference of confounding outside variables.
While these in vitro tests are not clinical measures of how the lens will perform when on the eye, there are studies that show an association of clinical on-eye lens surface wettability, lubricity, modulus, and Dk to that of comfort,4–7 making these at least valid characteristics to consider about a lens material.
There are also other in vitro measures used within the contact lens industry to show material property differences that are not so relevant. One such in vitro measure is lens pervaporation or lens evaporation.
Pervaporation (water transport through a lens to air) testing has occasionally been used incorrectly as a measure of lens evaporation (water transport from a lens to air) to show dehydration properties of a lens material.
Furthermore, controversary exists as to whether evaporation/dehydration is even an important differentiating material feature or not in modern hydrogel or silicone hydrogel lenses. Similar to the other in vitro measures, the testing conditions can be controlled so that each lens is treated in an identical manner to compare the lenses impartially.
However, the difference with this measure is that there has not been an association shown in literature of clinical on-eye lens dehydration to comfort.4,8,9 For this reason, in vitro lens dehydration measures are not an important characteristic to consider when trying to differentiate between contact lens material properties.
In Vitro Surface Wettability Testing
The most important distinguishing material features of water surface technology lenses are those at the surface of the lens and in particular, surface wettability. The surface wettability of a contact lens can be used to understand the wetting properties of a lens material and how it behaves in comparison with other lens materials. Alcon uses an in vitro method called interfacial dewetting and drainage optical platform (iDDrOP) to measure contact lens surface wettability to assess initial surface moisture break-up time to demonstrate how differently water surface technology lens materials behave compared with other traditional contact lenses.
This in vitro method is used to show surface wetting properties of each lens material while in an identical testing environment and is not a clinical performance assessment. With this method, lenses are rinsed, then soaked in phosphate buffered saline (PBS) for 16 hours to remove any blister pack solution from the lenses. The lenses are then mounted on a curved surface that maintains the shape of the contact lens and submerged into a PBS bath.
The lenses are then raised above the PBS surface while a video recording is taken of the surface of the lenses to visualise the dynamic fluid break-up at the lens surface. The first instance of a break in the lens surface fluid is reported as the surface moisture break-up time. Multiple measures for each lens type are taken, and the lenses are re-submerged in between measurements to fully rewet the lens before testing again.
Figure 2. In vitro average surface moisture break-up time per lens brand.
This testing method showed that Alcon’s Water Surface Technology lenses, Dailies Total1 and Precision1, demonstrated significantly longer surface moisture break up times than Bausch and Lomb Ultra One Day, Acuvue Oasys 1-Day, Biotrue ONEday, 1-Day Acuvue Moist, Clariti 1 Day, and MyDay lenses (p<0.001 for all) (Figure 2).10–12 A longer surface moisture break up time means that the surface moisture remained intact longer for Dailies Total1 and Precision1.10–12 The data demonstrates the unique in vitro material property difference of the two-phase water surface technology material compared with other traditional lens materials – the water surface technology material had the ability to hold moisture at the surface significantly longer. This in vitro measure is an important differentiating material characteristic that can only be understood by using in vitro testing and is one measure that helps to distinguish how fundamentally different the water surface technology material is in comparison with traditional soft contact lenses.
Dr Jessica Mathew OD PhD FAAO is Alcon’s Global Medical Affairs Director.
References
1. Thekveli, S., et al. Structure-property relationship of delefilcon A lenses. Cont Lens Ant Eye. 2012;35(supp 1):e14. 2.
2. Angelini, T., et al. Viscoelasticity and mesh-size at the surface of hydrogels characterized with microrheology. IOVS. 2013; 54. EAbstract 500.
3. Alcon data on file, 2019.
4. Dillehay, S., Does the level of available oxygen impact comfort in contact lens wear?: A review of the literature. Eye & Contact Lens: Science & Clinical Practice. 2007;33(3):148–155. DOI: 10.1097/01.icl.0000245572.66698.b1
5. Dumbleton, K., Guillon, M., Patel, T., et al.. Comfort and wettabiliity of daily disposable contact lenses. Poster presentation at TFOS Conference, 7–10 Sept 2016.
6. Fang, M., Airen, S., Jiang, H., Wang, J., Ocular surface microvascular response and its relation to contact lens fitting and ocular comfort: an update of recent research. Clin Exp Optom. 2021;104(6):661–671. DOI:10.1080/08164622.2021.1878867
7. Kern, J., Rappon, J., Bauman, E., Relationship between contact lens coefficient of friction and subjective lens comfort. Poster Presentation at the 31st Asia-Pacific Academy of Ophthalmology Congress, Taipei, Chinese Taipei, 24–27 March 2016.
8. Fonn, D., Hydrogel lens dehydration and subjective comfort and dryness ratings in symptomatic and asymptomatic contact lens wearers. Optom Vis Sci.1999;76(10):700–704. DOI: 10.1097/00006324-199910000-00021
9. Stapleton, F., Stretton, S., Papas, E., et al., Silicone hydrogel contact lenses and the ocular surface. Ocular Surface. 2006;4(1):25–43. DOI: 10.1016/s1542-0124(12)70262-8
10. Alcon data on file, 2019.
11. Alcon data on file, 2020.
12. Alcon data on file, 2022.
See instructions for use for wear, care, precautions, warnings, contraindications, and adverse effects.
†May use other proprietary wetting agents or other surface treatments to improve wettability.
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